1. Field of the Invention
The present invention relates to an improvement technique of an open/close valve, which is provided on an engine exhaust pipe outlet.
2. Description of Related Art
A boat that has an outlet of an exhaust pipe under water and allows the exhaust pipe to discharge exhaust gas into the water has been popularly used. To prevent water from entering an engine through the exhaust pipe when the boat is turned over, countermeasures have been proposed in, for example, Japanese Patent Application Laid-open No. 2004-98966 (see FIG. 8).
FIG. 8 in the JP 2004-98966 document is a perspective view showing one example of a valve, which is provided on the exhaust structure. The valve 60 (hereinafter referred to as an open/close valve 60) is provided in the vicinity of an exhaust outlet and is formed of a synthetic resin material. The valve 60 has a circular disc shape, wherein segment-like cuts are formed in the open/close valve such that the cut passes the center of the circular disc thus forming a plurality of triangular members 61. When the pressure in the inside of the exhaust pipe 38 is increased, a plurality of triangular members 61 is curved toward the exhaust downstream side. Therefore, the open/close valve 60 is opened in an approximately star shape thus allowing exhaust to be discharged to the outside of the boat to pass therethrough and preventing the intrusion of water from the outside of the boat. The open/close valve 60 performs the valve opening/closing operation by making use of the nature of the open/close valve 60. Specifically, the open/close valve 60 is deflected corresponding to the strength of an applied external force and restores an original state when the external force is removed.
The open/close valve 60 adopts the opening of the valve using the pressure of the exhaust gas as an operational principle thereof. Therefore, exhaust resistance is inevitably generated. When the exhaust resistance is large, the back pressure of the engine is increased thus adversely influencing the engine. When the open/close valve 60 is made easily deflectable to eliminate such an adverse influence, water is liable to easily enter the inside of the exhaust pipe.
Accordingly, there has been a demand for an open/close valve, which does not generate exhaust resistance in a usual state and makes the intrusion of water into the inside of the exhaust pipe difficult.